1. Field of the Invention
This invention relates to the field of ultrasonic imaging, and more specifically to the use of polymers as contrast agents for ultrasonic imaging.
2. Description of the Prior Art
There are a variety of imaging techniques that have been used to diagnose disease in humans. One of the first imaging techniques employed was X-rays. In X-rays, the images produced of the patient's body reflect the different densities of body structures. To improve the diagnostic utility of this imaging technique, contrast agents are employed in an attempt to increase the differences in density between various structures, such as between the gastrointestinal tract and its surrounding tissues. Barium and iodinated contrast material, for example, are used extensively for X-ray gastrointestinal studies to visualize the esophagus, stomach, intestines and rectum. Likewise, these contrast agents are used for X-ray computed tomographic studies to improve visualization of the gastrointestinal tract and to provide, for example, contrast between the tract and the structures adjacent to it, such as the vessels or the lymph nodes. Such gastrointestinal contrast agents permit one to increase the density inside the esophagus, stomach, intestines and rectum, and allow differentiation of the gastrointestinal system from surrounding structures.
Ultrasound is a more recent imaging technique which, unlike X-rays, does not utilize ionizing radiation. Instead, in ultrasound, sound waves are transmitted into a patient. These sound waves are then reflected from tissue in the patient and are received and processed to form an image. Since ultrasound does not employ ionizing radiation to produce these images, ultrasound is less invasive and safer to the patient than X-ray imaging techniques. Ultrasound, however, suffers at times in imaging clarity in comparison to X-rays, particularly where imaging of the gastrointestinal tract is involved. In ultrasound, one major problem is the presence of air/fluid interfaces, which results in shadowing of the ultrasound beam. Shadowing, in turn, prevents the ultrasound beam from penetrating beyond the air/fluid interface, and thus prevents visualization of structures near any air pockets. Another problem with ultrasound is the difficulty in imaging adjacent hypoechoic structures, that is, structures that are only minimally reflective of the ultrasound beam and, therefore, result in a low ultrasound signal. These problems are particularly evident in the gastrointestinal region, with its many air/liquid interfaces and its adjacent fluid-filled mucosal lining and often fluid-containing lumen. If better contrast agents were available for ultrasound, the diagnostic accuracy and the overall usefulness of ultrasound as an imaging modality, particularly in the gastrointestinal region, would be greatly enhanced.
In the past, investigators have attempted to solve the problems associated with gastrointestinal ultrasonic imaging by using water to fill the gastrointestinal tract. Water, however, was found to simply mix with the gas, and thus much of the shadowing resulting from the presence of air/fluid interfaces remained. In addition, the fact that water is absorbed by the bowel decreased its ability to serve any meaningful contrast enhancement function distally within the tract. Furthermore, the water is hypoechoic, and its presence adjacent to the fluid-filled hypoechoic mucosal lining of this region resulted in little differentiation of the tract lumen and from its lining. Intravenously administered glucagon has been employed in connection with such gastrointestinal imaging, since glucagon administered in this fashion relaxes the bowel by decreasing peristalsis. Although a helpful ultrasound adjunct, this, however, does not address such problems as shadowing caused by air/fluid interfaces and low differentiation caused by the presence of adjacent hypoechoic structures.
The need is great for contrast agents useful in ultrasonic imaging of various regions of the body, particularly those useful in imaging the gastrointestinal tract. The present invention is directed to these important ends.